The development of mobile telecommunications continues towards ever smaller and increasingly complicated handheld units or mobile phones. The development has recently led to new requirements for handheld units, namely that the units should support several different standards and telecommunications systems. Supporting several different systems requires several sets of filters and other radio frequency (RF) components in the RF parts of the handheld units. Despite this complexity, the size of a handheld unit should not increase as a result of such a wide support.
RF filters used in prior art mobile phones are usually discrete surface acoustic wave (SAW) or ceramic filters. This approach has been adequate for single standard phones, but does not allow support of several telecommunications systems without increasing the size of a mobile phone.
Surface acoustic wave (SAW) resonators utilize surface acoustic vibration modes of a solid surface, in which modes the vibration is confined to the surface of the solid, decaying quickly away from the surface. A SAW resonator typically comprises a piezoelectric layer and two electrodes. Various resonator structures such as filters are produced with SAW resonators. A SAW resonator has the advantage of having a very small size, but unfortunately cannot withstand high power levels.
It is known to construct thin film bulk acoustic wave (BAW) resonators on semi-conductor wafers, such as silicon (Si) or gallium arsenide (GaAs) wafers. For example, in an article entitled “Acoustic Bulk Wave Composite Resonators”, Applied Physics Letters, Vol. 38, No. 3, pp. 125-127, Feb. 1, 1981, by K. M. Lakin and J. S. Wang, an acoustic bulk wave resonator is disclosed which comprises a thin film piezoelectric layers of zinc oxide (ZnO) sputtered over a thin membrane of silicon (Si). Further, in an article entitled “An Air-Gap Type Piezoelectric Composite Thin Film Resonator”, 15 Proc. 39th Annual Symp. Freq. Control, pp. 361-366, 1985, by Hiroaki Satoh, Yasuo Ebata, Hitoshi Suzuki, and Choji Narahara, a BAW resonator having a bridge structure is disclosed. Examples of BAW resonator circuits are also disclosed in EP-A-0962999 and EP-A-0834989.
BAW resonators are not yet in widespread use, partly due to the reason that feasible ways of combining such resonators with other circuitry have not been presented. However, BAW resonators have some advantages as compared to SAW resonators. For example, BAW structures have a better tolerance of high power levels.
FIG. 1 shows a cross section of a conventional BAW resonator isolated from a substrate 30 (e.g. an Si-substrate) by an acoustic mirror structure 18. The BAW resonator comprises a bottom electrode BE, a piezoelectric layer or film 160, and a top electrode TE. The acoustical mirror structure 18 comprises in this example three layers. Two of the layers are formed of a first material, and the third layer in between the two layers is formed from a second material. The first and second materials have different acoustical impedances. The order of the materials can be different in different examples. In some examples, a material with a high acoustical impedance can be in the middle and a material with a low acoustical impedance on both sides of the middle material. In other examples, the order can be opposite. The bottom electrode BE may in some embodiments function as one layer of the acoustical mirror.
In FIG. 1, the active part 16 of the BAW resonator is indicated by the dashed rectangle. This BAW resonator is based on an SMR (Solid Mounted Resonator) structure, where reflection is made by the mirror structure 18 under the active part. The electronic characteristic between the bottom electrode BE and the substrate 30 can be represented by a bottom electrode parasitic circuit BEP which comprises a series connection of parasitic capacitors CoxM1 to CoxM3 at the acoustical mirror structure 18, followed by a parallel circuit of a substrate resistor RsuM and a substrate capacitor CsuM. Furthermore, resistors Rsb and Rst represent ohmic resistances of the respective conductor paths between the bottom electrode BE and a bottom electrode terminal 24 and between the top electrode TE and a top electrode terminal 22. The electronic characteristic between the top electrode TE and the substrate 30 can be represented by a top electrode parasitic circuit TEP which comprises a series connection of a parasitic capacitor CoxT and a parallel circuit of a substrate resistor RsuT and a substrate capacitor CsuT. Moreover, a parasitic capacitance Ctb is provided between the top electrode TE and the bottom electrode BE. Thus, the electrodes of the conventional BAW resonator are slightly different because the bottom electrode BE has more parasitic capacitance than the top electrode TE.
The temperature drift of BAW resonators is approx. −20 ppm/° C. The usable mobile temperature range is −30° C. to +85° C., so that frequency drift could be as much as 2000 ppm respectively. If the center frequency of oscillator is for example 1 GHz, then the drift will be 2.0 MHz.
Quite many implementations need more accurate frequency than referred to above, such as for example in case of a reference oscillator adapted for mobile use. Temperature compensation can be achieved, if the dependency or relationship between frequency and temperature is well known. However, when a mobile phone is switched on, temperature and oscillator frequency are unknown parameters.
Document US2005/0110598A1 discloses a temperature-compensated FBAR device, where an integrated temperature-compensating element having a temperature coefficient opposite in sign to the temperature coefficient of a piezoelectric element of the active part is provided for temperature compensation purposes. Additionally, document U.S. Pat. No. 6,710,508B2 discloses an FBAR device in which resonant frequencies are adjusted by intentionally inducing oxidation at an elevated temperature.
Furthermore, different separate temperature control components, not integrated with an acoustic resonator of an SAW oscillator to be compensated, are described in “Low Noise, Low Jitter Hybrid Ovenized SAW Oscillators”, J. V. Adler et al, IEEE ULTRASONICS SYMPOSIUM, pp 25-28, 2000.